In case of a vehicle rear impact in a situation where an upper half of the body of an occupant has not been supported by a seatback, a sudden movement of the upper body to the seatback triggers a head of the occupant to be unexpectedly moved to a headrest. Here, a difference is created between a movement amount of the upper body and the one of the head. More specifically, the head of the occupant is moved to in a rearward direction of a vehicle farther than the upper body. Subsequently, the upper body is swung widely toward the front in response to a reaction of the impact force. Here, the neck of the occupant bears a load due to the difference in the movement amount.
In the light of the foregoing, conventionally, headrest control apparatuses have been suggested, each of which is provided with a mechanism for moving a headrest to a position of a head of an occupant in case of a vehicle rear impact. According to this structure, a head of an occupant is restrained from widely moving rearward in case of a rear impact and is protected, thereby reducing a load subjected to the neck of an occupant.
In order to stop appropriately a headrest at a position of a head of an occupant with the aforementioned structure of the apparatus, it is possible to provide a proximity sensor, such as a capacitive sensor, at the headrest and to detect an approaching of the head towards the headrest. A structure of a capacitive detecting circuit, which detects a detected object, is disclosed for example in JP2002-014174A, in which the approaching of the head is detected based upon oscillation of an oscillating circuit, which oscillates in response to changes in voltage due to electric charging and discharging to a sensor electrode for example. Further, such structure of a capacitive detecting circuit is further disclosed for example in JP2001-178136A, in which capacitance of a sensor electrode is compared with reference capacitance of a control circuit and an approaching of the head is detected based upon the comparison result.
In such headrest control apparatus, in order to detect a head of an occupant while moving the headrest forward, such conventional capacitive detecting circuit electrically charges and discharges the sensor electrode by a switching operation of a clock signal with a high frequency. Therefore, switching noise of a clock signal is created and noise may be released from a radio mounted on a vehicle. That is, there is a possibility that this sensor electrode serves as an antenna and radio noise is released. Further, where a sensor, which is different from a capacitive sensor, is employed as a proximity sensor, when an ultrasonic sensor is employed for example, an occupant may recognize supersonic waves, which are emitted from the ultrasonic sensor, as noise. Further, there is a case that a headrest is equipped with a contact sensor to detect a contact of a head against the headrest. In such case, it is possible to employ a piezoelectric sensor as a contact sensor. A piezoelectric element of the piezoelectric sensor is applied with alternating voltage with a predetermined frequency and is oscillated with a predetermined frequency. Once a head of an occupant makes a contact with the headrest, the piezoelectric sensor detects reduction in amplitude of the frequency and confirms the contact of the head against the headrest. However, in such circumstances, because alternating voltage with a predetermined frequency is applied to the piezoelectric element, noise may be generated based upon this alternating voltage.
The present invention has been made in view of the above circumstances, and provides a headrest control apparatus for a vehicle, which apparatus detects a proximity of a head towards a headrest by use of a proximity sensor, a contact sensor, or the like and restrains an occurrence of noise such as radio noise, and a method of controlling the headrest control apparatus.